One way of transferring a pattern on to a surface by e-beam lithography is to use a variable shaped beam or VSB to reveal a positive or negative resist coating. For doing so, during a fracturing step, the pattern is to be cut into elementary forms to which a radiation dose is assigned (known as “shots”). Geometry and radiation dose of the shots are closely interrelated because, at the dimensions which are now used (technologies with a critical dimension or “CD” of less than 60-80 nm), the proximity effects (forward scattering and backward scattering) largely depend on the density of the exposed area.
The patterns to be transferred are quite often of a simple geometric form, such as thin rectangles (lines) or squares (interconnections). In these circumstances, the geometry of the shots is defined accordingly and is also simple: each pattern is fractured into a union of rectangular or square shots.
Nevertheless, for a number of applications (inverse lithography, photonics, metrology calibration, source mask optimization, etc. . . . ), it may be necessary or advantageous to include in the design patterns which are not simple forms of the type previously described, but which may be circles or of an indeterminate, possibly curvilinear, form (further referred to as free-form).
Under these circumstances traditional fracturing is not advantageous because it generates a very high number of shots, especially when pattern fidelity is critical. The writing time increases proportionally to the number of shots, which in turns increases significantly the cost of producing masks or wafers.
It would therefore be advantageous to use a fracturing method capable of adapting itself to indeterminate forms of patterns to be transferred onto a surface. It would be advantageous to be able to use circular shots to fracture free forms.
An attempt in this direction is discussed in U.S. Pat. No. 8,057,970 which discloses an assembly method for forming circular patterns using a plurality of overlapping VSB shots with varying dosages and various successive combinations thereof.
But this prior art document does not properly address the problem of controlling the roughness of the contour of the free form which is actually exposed and the exposure dose.